1. Field of the Invention
The present invention relates to a control device for an exhaust gas sensor of internal-combustion engine, and more particularly to a control device suitable for controlling an exhaust gas sensor that is equipped with a sensor element having a function for pumping oxygen in a gas.
2. Background Art
A device for controlling an air-fuel ratio sensor, which is positioned in an exhaust path of an internal-combustion engine, is disclosed by Japanese Patent Laid-open No. Hei 9-101285. This device applies a predetermined positive voltage to the air-fuel ratio sensor's sensor element during an internal-combustion engine's operation. In this instance, the sensor element pumps oxygen in an exhaust gas, and causes an electrical current to flow in accordance with the amount of such pumping, that is, invokes the flow of a limiting current corresponding to the oxygen concentration in the exhaust gas. The oxygen concentration in the exhaust gas corresponds to the air-fuel ratio of the exhaust gas. Therefore, when the above-mentioned conventional configuration is employed, the air-fuel ratio of the exhaust gas can be detected in accordance with the electrical current flow in the sensor element.
The above sensor element has an electrode that is exposed to an exhaust gas and an electrode that is exposed to atmospheric air. When the sensor element is used, oxygen is combined with the electrode that is exposed to atmospheric air. When the degree of such combination increases, the resistance of the sensor element increases, then the characteristic of the air-fuel ratio sensor deteriorates. To achieve recovery from such deterioration, the above conventional device applies a negative voltage, which is oriented in a direction opposite to that of a normally applied positive voltage, to the sensor element during fuel cut of an internal-combustion engine. While the fuel cut is being performed, there is no need to detect the air-fuel ratio. Therefore, a negative voltage can be applied to the sensor element at such timing without decreasing the control accuracy of the air-fuel ratio.
When a negative voltage is applied to the sensor element that is adequately warmed up, the oxygen combined with the electrode exposed to atmospheric air leaves the electrode and moves toward the other electrode. Therefore, when the above conventional configuration is employed, the sensor element can achieve recovery as needed from deterioration during an internal-combustion engine operation, thereby making it possible to maintain adequate detection accuracy of the air-fuel ratio sensor.
It is generally known that the air-fuel ratio sensor element and the like generate a stable output when heated to a predetermined activity temperature after internal-combustion engine startup. To obtain an excellent emission characteristic in an internal-combustion engine, it is preferred that the time interval between the instant at which the internal-combustion engine is started and the instant at which the exhaust gas sensor generates a stable output be minimized. However, the conventional device described above attempts to achieve sensor characteristic restoration in synchronism with fuel cut sequence execution after the end of internal-combustion engine warm-up. That is, the conventional device does not reduce the period of time that is required for the exhaust gas sensor to generate a stable output after the internal-combustion engine startup.